This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application entitled Horizontal Position Control Circuit of a Display Apparatus filed with the Korean Industrial Property Office on Sep. 9, 1997 and there duly assigned Serial No. 97-46248 by that Office.
1. Field of the Invention
The present invention relates to a horizontal position control circuit of a display device and, more particularly, to a horizontal position control circuit having a horizontal position control function by which a displayed image can be positioned horizontally (centered) on a monitor screen regardless of the frequency of the horizontal sync signal provided by a video card located, typically, in the main body of a host computer.
2. Description of the Related Art
In a CRT-type display device, a deflection circuit controls electron beam deflection so that images are formed on a screen through an electrostatic deflection, using yokes (coils) mounted on the CRT. The images are displayed on the screen in accordance with sawtooth waveform current signals respectively applied to horizontal and vertical coils.
As shown in FIG. 1, a conventional horizontal display positioning circuit is largely comprised of: a microcomputer 20 for discriminating various display modes based on horizontal and vertical sync signals received from a host computer (not shown) and, in accordance with the discriminated mode, outputting a pulse width modulated (PWM) control signal and a horizontal sync output signal; a horizontal deflection circuit 40 and a horizontal deflection yoke 60. The horizontal deflection circuit 40 includes a horizontal oscillating circuit 41, which generally includes a blocking oscillation circuit and has position control circuitry (not shown), wherein the horizontal oscillating circuit 41 generates a horizontal deflection signal based upon the PWM control signal and the horizontal sync output signal; a horizontal drive circuit 42 which performs waveform correction of the horizontal deflection signal and drives an output transistor (of the next stage); and a horizontal output circuit 43 which produces a sawtooth current waveform through the horizontal deflection coil (yoke) 60 based on the operation of the output transistor of the horizontal output circuit 43. A resistor R3 is connected to a voltage Vcc and provides for user (manual) adjustment of the horizontal display.
As shown in FIGS. 1 and 2A, the microcomputer 20 generates a horizontal sync output signal as a positive-going value based on the horizontal sync signal from the host computer. The horizontal sync output signal is filtered through an RC network having a resistor R2 and a capacitor C2 and the filtered horizontal sync output signal is applied as a horizontal sync input signal to the horizontal oscillating circuit 41.
To control the screen position of a displayed image horizontally, the horizontal oscillating circuit 41 uses the center point of the xe2x80x9conxe2x80x9d cycle of the PWM control signal as a reference, as shown in FIG. 2B. Here, it should be noted that the polarity of the PWM waveform, i.e., whether it be active negative or active positive, is fixed without regard to the polarity of the horizontal sync signal as supplied by the host computer. Actually, a resistor R1 and smoothing capacitor C1 are used to xe2x80x9crectifyxe2x80x9d the PWM control signal from the microcomputer 20, and the resistor R3 adjusts the rectified PWM control signal to generate a position control signal which is input to the horizontal oscillating circuit 41, thereby enabling a user (manual) adjustment of the horizontal display.
Once a displayed image is positioned at the horizontal center point of a monitor screen according to the monitor""s video reference mode, should the video mode become redefined, the image is displayed with a new center point, i.e., one that is shifted by an amount corresponding to the change in duty width of the incoming horizontal sync signal. That is, if a horizontal sync signal from the host computer and having an increased duty width is input to the microcomputer 20, the PWM control signal, as shown in FIG. 2C, exhibits a greater duty width, which is represented by a gap g.
To maintain the position of the displayed image at the center point of the screen, the microcomputer 20 computes a horizontal position control (compensation) signal based on the horizontal sync signal input thereto and outputs the result as the PWM waveform. This output is the result of a microcomputer calculation based on the frequency of the incoming horizontal sync signal, to thereby produce the PWM control signal having a duty ratio reflecting the xe2x80x9cscreen shiftxe2x80x9d carried out by the position control circuitry (not shown) of the horizontal oscillating circuit 41. The amount of screen shift corresponds to the size of the gap g in FIG. 2C.
The horizontal oscillation circuit 41 superimposes the PWM waveform onto the horizontal sync input signal, thereby producing a compensating current value. Here, the PWM waveform has a duty width proportionate to the degree to which the screen is horizontally shifted in accordance with the duty width variation (gap) of the horizontal sync signal of a given video mode.
Accordingly, for anticipated display modes, i.e., those considered at the time of monitor design, a displayed image can be accurately positioned (centered) on the screen independent of duty width variations in the incoming horizontal sync signal, using the monitor""s reference mode to center the image display.
However, if a horizontal sync signal having a frequency not considered at the time of monitor design is input, the screen position which is thus shifted by as much as the gap of the unexpected sync signal""s duty width is not compensated by the PWM control signal from the microcomputer 20, thereby resulting in an undesired shifting of the displayed image to the left or right of center. This also gives rise to video card compatibility problems and impedes manual efforts (user adjustments) to center an image display in the screen horizontally.
Accordingly, in order to overcome such drawbacks in the conventional art, it is therefore an object of the present invention to provide a horizontal positioning circuit which enables horizontal centering of a displayed image independently from the duty width of the incoming horizontal sync signal.
It is another object of the present invention to provide a horizontal positioning circuit which enables horizontal centering of a displayed image independently from the polarity of the incoming horizontal sync signal.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided a horizontal display positioning circuit for a CRT-type display device, including a microcomputer, which receives synchronizing signals from a host computer and discriminates a video mode according to the received synchronizing signals, for outputting a first horizontal sync signal having a given polarity; a horizontal deflection circuit; and a horizontal position control circuit, configured between the microcomputer and the horizontal deflection circuit, for outputting a second horizontal sync signal having a predetermined duty width.